Tube pump and liquid ejecting apparatus

ABSTRACT

A tube pump including a housing with a cylindrical inner circumferential surface, wherein a portion of a flexible tube is wound with an upstream portion and a downstream portion of the tube overlapping at a overlapping section, and a pressing member capable of pressing the tube which is movably housed along the inner circumferential surface. The housing includes a relief portion for releasing either the upstream portion or the downstream portion from the pressing of the pressing member, wherein the relief portion is formed on the inner circumferential surface at a position which corresponds to the position of tube overlapping section.

BACKGROUND OF THE INVENTION

The entire disclosure of Japanese Patent Application No. 2006-236910, filed Aug. 31, 2006 is expressly incorporated herein by reference.

1. Technical Field

The present invention relates to a liquid ejecting apparatuses. More specifically, the present invention relates to a tube pump for a liquid ejecting apparatus.

2. Related Art

One example of a liquid ejecting apparatus is a ink jet printer (hereinafter, referred to as “a printer”), which ejects a liquid onto a target from a nozzle formed in a liquid ejecting head. In most printers, the nozzle of the print head (liquid ejecting head) is open, meaning that ink solvent in the nozzle may evaporate, causing ink ejection failure as the ink viscosity increases. In response to this problem, printers generally include a head maintenance mechanism.

Generally, the head maintenance mechanism includes a cap which is used to seal the surface of the nozzle and a sucking pump located in an ink discharging passage which communicates with the cap. The head maintenance mechanism prevents failed ink ejection by using the sucking pump to generate a negative pressure in the cap in order to remove the ink with increased viscosity from the nozzle. Typically, the sucking pump uses a tube pump, such as the tube pump disclosed in Japanese Patent Application No. JP-A-2001-301195.

The tube pump disclosed in JP-A-2001-301195 includes a substantially cylindrical housing, referred to as a “guide member,” for holding a middle portion of a flexible tube. The guide member includes a rotating body which rotates about an axis of the guide member and a roller or pressing member which is capable of pressing the tube. The roller moves along an inner circumferential surface of the guide member when the rotating body rotates in a predetermined direction.

The portion of flexible tubing is wound around the inner circumference of the guide member into a circular shape (α shape). That is, the tube is arranged such that an upstream portion and a downstream portion of the tube partially overlap each other.

Accordingly, when the rotating body operates by using a roller, which moves along the inner surface of the guide member in order to press portions of the tube against the inside surface of the guide member. Using this method, the guide member is able to depressurize the upstream portion of the tubing, creating a negative pressure in the cap, which causes the ink of increased viscosity to be sucked out of the nozzle.

One difficulty of the tube pump disclosed in JP-A-2001-301195, however, is that a portion of the tube is wound so as to have an α shape in the housing, so that there is a portion of the upstream portion and the downstream portion that overlap each other. At times, the overlapping portions of the tube may be squashed by the roller. In order to squash the overlapped portion, a pressing force that is twice the force required to press a single portion of tubing is needed. In order to provide the requisite amount of force to press the overlapping section of the tube, a motor is required that is capable of enhancing pump torque and handling the load variation must be used. As a result, the motor is more expensive and larger, and may have difficulties stably driving the tube pump.

BRIEF SUMMARY OF THE INVENTION

One advantage of some aspects of the invention is a tube pump for a liquid ejecting apparatus with reduced pump torque and pump load variation.

One aspect of the invention, is a tube pump which includes a housing with a cylindrical inner circumferential surface wherein a portion of flexible tube is wound around the cylindrical inner circumferential surface such that an upstream portion and a downstream portion of the tube partially overlap each other at a overlapping portion. The tube pump further comprises and a pressing member for pressing the flexible tube which is movably housed along the inner circumferential surface of the housing, wherein the housing includes a relief portion for releasing either the upstream portion or the downstream portion from the pressing of the pressing member.

Another aspect of the invention is a liquid ejecting apparatus including a liquid ejecting head capable of ejecting a liquid from a nozzle formed on a nozzle formation surface; a cap capable of sealing the nozzle formation surface; and a sucking mechanism that capable of removing the liquid from the cap, wherein the sucking mechanism includes the tube pump described above.

By using relief portions which correspond to the tube overlapping position in the inner circumferential surface of the housing, either the upstream portion or the downstream portion is moved to the relief portion, releasing one portion of the overlapping section from the pressing of the pressing member. As a result, it is possible to reduce the load required for the pressing member to compress the overlapping portion, thereby reducing pump torque and load variation in the pump.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.

FIG. 1 is a perspective view illustrating an ink jet printer according to an embodiment of the invention;

FIG. 2 is a schematic sectional view illustrating the major components shown in FIG. 1;

FIG. 3 is a perspective view illustrating a tube pump;

FIG. 4 is an exploded perspective view of the tube pump;

FIG. 5 is a top sectional view of the tube pump;

FIG. 6 is an exploded perspective view of a pressing member which is assembled into a pump foil; and

FIG. 7 is a side view illustrating a pressing member.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a tube pump will be described which is mounted in an ink jet printer, as one example of a liquid ejecting apparatus capable of performing the invention, will be described with reference to the drawings. In addition, the terms “front,” “rear.” “up,” “down,” “right,” and “left” correspond to the directions shown in FIG. 1, unless otherwise mentioned.

FIG. 1 shows an ink jet printer 11, which includes a frame 12 which is substantially shaped into a rectangular box. A platen 13 is longitudinally arranged in the right and left directions of the lower portion of the frame 12. Print paper P is configured to be fed on the platen 13 from a rear side by a paper feeding mechanism (not shown) when driven by a paper feeding motor 14 provided in the lower rear surface of the frame 12.

A guide shaft 15 is arranged in longitudinal directions of the platen 13 in the upper portion of the platen 13 of the frame 12. A carriage 16 is supported in the guide shaft 15, so as to reciprocate along the axis (the right and left direction) of the guide shaft 15. That is, the guide shaft 15 is inserted into a supporting hole 16 a formed in the right and left direction, and the carriage 16 is supported so as to reciprocate along the longitudinal directions of the guide shaft 15.

A driving pulley 17 a and a follower pulley 17 b are supported in the inner rear surface of the frame 12, so as to allow them to rotate corresponding to the end portions of the guide shaft 15. An output shaft of a carriage motor 18 is used as a driving source for enabling reciprocation of the carriage 16, and is connected to the driving pulley 17 a. An endless timing belt 17 is connected to the carriage 16 and is arranged between the pair of the pulleys 17 a and 17 b. Accordingly, the carriage 16 is guided along the guide shaft 15 in the right and left direction by using the endless timing belt 17 with the drive of the carriage motor 18.

A print head 19 which acts as a liquid ejecting head is provided in the lower surface of the carriage 16. An ink cartridge 20 for supplying ink to the print head 19 is detachably mounted on the carriage 16. In addition, the ink in the ink cartridge 20 is supplied to the print head 19 from the ink cartridge 20 by driving the piezoelectric elements 21 (see FIG. 2) included in the print head 19. The ink is ejected from a plurality of nozzles 22 (see FIG. 2) included in the print head 19 to the print paper P which is fed to the platen 13. In this way, the printing process is performed.

A maintenance mechanism 23 is provided to carry out maintenances such as cleaning of the print head 19 during a non-printing period. The maintenance mechanism 23 is located at a home position area which is a non-printing area in the right end portion of the frame 12, and is described in more detail below.

As shown in FIG. 2, the maintenance mechanism 23 includes a rectangular cap 24 having a bottom surface and an elevating device 25 for elevating the cap 24. In addition, the maintenance mechanism 23 allows the elevating device 25 to elevate the cap 24 when the carriage 16 is moved to the home position area so that the cap 24 seals the nozzle formation surface 19 a of the print head 19. Moreover, a protruding portion 26 which protrudes from the bottom wall of the cap 24. In addition, in the protruding portion 26, a discharging passage 26 a for discharging the ink from the cap 24 is formed in the up and down direction.

One end (upstream side) of the discharging tube 27 made from a flexible material is connected to the protruding portion 26 and the other end (downstream side) of the discharging tube 27 is connected to the inside of a rectangular used ink tank 28. At a portion of the discharging tube 27 about halfway between the cap 24 and the used ink tank 28 is a tube pump 29 which acts as a sucking mechanism for transferring the liquid from the cap 24 to the used ink tank 28.

The tube pump 29 is driven while the nozzle formation surface 19 a (each of the nozzles 22) of the print head 19 is sealed by the cap 24. During this time, the cleaning process is carried out, meaning that the ink with increased viscosity and any bubbles are sucked from each of the nozzles 22 and discharged to the inside of the used ink tank 28 via the discharging tube 27. Moreover, a used ink absorber 30 for absorbing and maintaining the ink discharged to the used ink tank 28 is located in the used ink tank 28.

As shown in FIGS. 3 and 4, the tube pump 29 includes a cylindrical housing 31 with a bottom surface fixed to the inside of the frame 12 (see FIG. 1). A hole 31 a is formed through the center of the bottom surface of the housing 31. In the housing 31, a pump foil 32, which acts as a rotating body, is housed so as to rotate on an axis A passing through the center of the housing 31. The pump foil 32 extends along the axis A and includes a foil shaft 33 that is inserted into the hole 31 a. Moreover, the pump foil 32 is configured so as to rotate on the foil shaft 33 in the housing 31.

An upstream opening 34 and a downstream opening 35 are located so as to oppose each other on the inner surface 31 b of the housing. In this case, the upstream opening 34 and the downstream opening 35 do not lie along the axis A. In addition, a portion 36 of the discharging tube 27 is housed in the housing 31 so as to wind along the inner circumferential surface 31 b of the housing 31 through the upstream opening 34 and the downstream opening 35.

In this case, a portion 36 of the upstream portion 36 a and downstream portion 36 b of the discharging tube 27 housed in the housing 31 overlaps, and is referred to as a tube overlapping portion B. Moreover, in the embodiment, the portion 36 a of the discharging tube 27 in the housing 31 is configured to be wound at 360° so as to minimize the tube overlapping portion B.

The tube overlapping portion B includes a thin portion 37, so that the thickness of the discharging tube 27 of the thin portion 37 is thinner than that of other portions of the discharging tube 27 in the housing 31. In this case, the thickness of the thin portion 37 is configured so that the tube overlapping portion B is as flexible and easily compressed as other portions (where one tube is curved) in the midway portion 36 of the discharging tube 27.

As shown in FIG. 5, an upstream concave portion 38 and a downstream concave portion 39, is formed in the inner circumferential surface 31 b of the housing 31. The upstream concave portion 38 and the downstream concave portion 39 constitute a relief area where the upstream portion 36 a and the downstream portion 36 b of the overlapping portion B can be housed, respectively. In this case, the upstream concave portion 38 and the downstream concave portion 39 are arranged adjacently so as to extend along the inner circumferential surface 31 b of the housing 31 in a top view.

As shown in FIG. 6, the pump foil 32 includes a large plate 40 and a small plate 41, wherein the diameter the small plate 41 is smaller than that of the large plate 40. In addition, the large plate 40 and the small plate 41 are attached to a foil shaft 33 at a predetermined distance from each other with the foil shaft passing through each center thereof. A circular arc-shaped roller guide groove 42 is formed in the large plate 40. One end of the roller guide groove 42 is positioned such that one end of the grove is near the outer circumference of the large plate 40 and the other end is closer to the inner circumference of the large plate 40. That is, the roller guide groove 42 becomes gradually further away from the outer circumference of the large plate 40 at one end.

A roller guide concave portion 43 is a notch in the periphery of the small plate 41, which corresponds to the roller guide groove 42 of the large plate 40. In addition, cam surfaces C are formed in each of the inner surfaces of the roller guide groove 42 of the large plate 40 and roller guide concave portion 43 of the small plate 41. A roller 44 is movably inserted into the roller guide groove 42 through a shaft 45. Thus, the roller 44 is configured to slide the shaft 45 along the cam surfaces C through so that both ends of the shaft 45 come in contact with the cam surfaces C. Moreover, the roller 44 and the shaft 45 comprise a pressing member 46.

As shown in FIG. 7, the roller 44 and the shaft 45 comprise separate components of the pressing member 46. Thus, a shaft inserting hole 44 a is formed through the axis of the roller 44. In addition, the pressing member 46 is configured so that the shaft 45 is inserted through the shaft inserting hole 44 a. Accordingly, the shaft 45 is inserted through the shaft inserting hole 44 a of the roller 44 so that both the ends of the shaft 45 protrude from the shaft center of the roller 44 the in longitudinal direction. In this case, the roller 44 and the shaft 45 are capable of individually rotating.

The shaft 45 of the pressing member 46 is made from a material that has a lower frictional resistance than the roller 44. Specifically, a metal or a synthetic resin with a low frictional resistance is used. Examples of the synthetic resin that may be used include sliding grades such as polyacetal (POM) or polystyrene (PS). In the embodiment, as the material of the shaft 45 of the pressing member 46 is polyacetal of a sliding grade. Accordingly, the shaft 45 has a lower frictional resistance than the roller 44.

In both portions where the shaft 45 extends beyond the roller 44, separation preventing pins 47 are mounted in order to prevent the shaft 45 from being separated from the shaft inserting hole 44 a of the roller 44. The separating preventing pins 47 are configured so as not to interfere with the individual rotation of the roller 44 and the shaft 45.

When a pump motor (not shown) is driven and the pump foil 32 is rotated in a pump operating direction (along the direction of the arrow shown in FIG. 5), the pressing member 46 moves to one end of the roller guide groove 42. The movement allows the pressing member 46 to compress the midway portion 36 of the discharging tube 27 from the upstream side to the downstream side as the pressing member 46 rotates.

The rotation of the pressing member 46 causes the discharging tube 27 on the upstream side to depressurize. Because the air or ink in the cap 24 is gradually discharged toward the used ink tank 28 by rotating the pressing member 46 in the pump operating direction of the pump foil 32, a negative pressure is generated in the inside of the cap 24.

Alternatively, when the pump foil 32 is rotated in a direction that is opposite the pump operating direction, the pressing member 46 moves to the other end of the roller guide groove 42. This allows the pressing member 46 to lightly compress the midway portion 36 of the discharging tube 27 and pressurizes the inside of the discharging tube 27. That is, when the pump foil 32 is rotated in the pump non-operating direction, the discharging tube 27 on the pressurized on the upstream side. Moreover, the pressing member 46 presses the portion 36 of the discharging tube 27 so that no negative pressure is generated in the cap 24.

As shown in FIG. 6, when the pressing member 46 moves to the one end of the roller guide groove 42 of the pump foil 32 to the shaft sliding contact portion N, the pressing member 46 comes into contact with the discharging tube. In one embodiment, the pressing member 46 is formed of a low-friction material so as to have a lower frictional resistance than other portions. In the embodiment, as the low-friction material, polytetrafluoroethylene (PTFE) is used.

Next, an operation of the tube pump 29 will be described in more detail. During the cleaning process of the print head 19, each of the nozzles 22 of the print head 19 is sealed by the cap 24. When the pump motor (not shown) is driven to rotate the pump foil 32 in the pump operating direction, the roller 44 rotates on the shaft 45 and moves to compress the middle portion 36 of the discharging tube 27 from the upstream side to the downstream side. During this time, both the ends of the shaft 45 rotate and slide to the shaft sliding contact portions N of the cam surfaces, and rotate the roller 44.

In this case, the shaft sliding contact portions N in the cam surfaces C are formed of polytetrafluorethylene (the low-friction material) and the shaft 45 is formed of polyacetal (synthetic resin having low friction) of sliding grade. Accordingly, a frictional resistance between the shaft 45 and the cam surfaces C and the shaft 45 and the roller 44 are decreased.

When the roller 44 presses the tube overlapping portion B, the upstream portion 36 a and the downstream portion 36 b move to the upstream concave portion 38 and the downstream concave portion 39, respectively, so as to be released from the compression of the roller 44. Moreover, due to the thinned portion 37 of the tube overlapping portion B, the tube overlapping portion B may be compressed easily as other portions of the discharging tube 27. As a result, the pump torque remains consistent when the roller 44 presses the tube overlapping portion B.

Subsequently, when the roller 44 rotates to compress the middle portion 36 of the discharging tube 27 from the upstream side to the downstream side, the discharging tube 27 on the upstream side is depressurized. Accordingly, the negative pressure is generated in the cap 24. The negative pressure causes the air or ink in each of the nozzles 22 or the cap 24 to discharge into the used ink tank 28 through the discharging tube 27, ending the cleaning process of the print head 19.

Some advantages of the above-described embodiment are:

(1) Sliding grade polyacetal is used as the material of the shaft 45, such that the shaft 45 has a lower frictional resistance than the roller 44. Because the frictional resistance between the shaft 45 and the cam surfaces C is deceased, the amount of force required to rotate the pump foil 32 in pump operating time is reduced. Accordingly, since the pump torque can be reduced, the pump motor (not shown) for driving the pump foil 32 can be smaller. As a result, it is possible to improve the efficiency of the tube pump 29.

(2) Since the roller 44 and the shaft 45 constituting the pressing member 46 are separate components, the quality of the material of the shaft 45 and the roller 44 can be easily changed depending on specifications of the tube pump 29.

(3) The shaft 45 and the roller 44 of the pressing member 46 are configured to individually rotate. Thus, the frictional resistance between the shaft 45 and the roller 44 is lower than that between the shaft 45 and the cam surfaces C, it is possible to rotate the roller 44 in pump operating time. For this reason, the friction between the shaft 45 and the cam surfaces C can be suppressed. As a result, it is possible to prevent the shaft 45 and the cam surfaces C from abrading.

(4) Using the separation preventing pin 47 to prevent the roller 44 from separating from the shaft 45, it is possible to maintain the position of the shaft 45 and the roller 44.

(5) The shaft sliding contact portions N in the cam surfaces C of the pump foil 32 are formed of polytetrafluoroethylene so as to have a lower frictional resistance than other portions of the cam surfaces C. When the pressing member 46 presses the middle portion 36 of the discharging tube 27, the sliding resistance between the shaft 45 and the cam surfaces C can be reduced. Thus, the abrasion of the cam surfaces C can be reduced and the load at the time of driving the tube pump 29 can be reduced, making it possible to reduce the pump torque and improve the pump efficiency.

(6) Since the pressing member 46 presses two discharging tubes 27 (the upstream portion 36 a and the downstream portion 36 b), a larger load is required to compress the tube overlapping portion B than other portions discharging tube 27. By using a thinner portion 37 in the tube overlapping portion B, the tube overlapping portion B is more flexible and easily compressed than other portions in the midway portion 36 of the discharging tube 27. Thus, the load required to compressed the tube overlapping portion B can be reduced. As a result, it is possible to reduce the pump torque and improve the pump efficiency.

(7) The discharging tube 27 is configured so that the tube overlapping portion B can be compressed as easily as other portions of the discharging tube 27 housed in the housing 31. That is, the discharging tube 27 is configured so that the load required for the pressing member 46 to compress the tube overlapping portion B is almost equal to the load required for the pressing member 46 to compress other portions of the discharging tube 27. For this reason, the load required to compress the portion 36 of the discharging tube 27 in the housing 31 is uniform across the whole circumference of the discharging tube 27 in the housing 31. As a result, the variation in the pump torque is suppressed, and it is possible to stably drive the tube pump 29.

(8) Since the pressing member 46 presses the two discharging tubes 27 (the upstream portion 36 a and the downstream portion 36 b), a larger load is required to compress the tube overlapping portion B than other portions of the discharging tube 27 in the housing 31. However, according to the embodiment, the upstream concave portion 38 and the downstream concave portion 39 are formed in the inner circumferential surface 31 b of the housing 31 so as to correspond to the upstream portion 36 a and the downstream portion 36 b, respectively. For this reason, when the pressing member 46 presses the tube overlapping portion B, the upstream portion 36 a and the downstream portion 36 b move to the upstream concave portion 38 and the downstream concave portion 39, respectively, so as to release the discharging tube 27 from the pressing of the pressing member 46. Accordingly, it is possible to effectively reduce the load required to compress the tube overlapping portion B, and to thus reduce the pump torque and the variation in the pump load. As a result, when the print head 19 is cleaned, the tube pump 29 can remove the liquid in the cap 24 effectively and stably.

(9) Since the upstream concave portion 38 and the downstream concave portion 39 in the housing 31 are formed along the inner circumferential surface 31 b of the housing 31, the tube overlapping portion B is minimized. For this reason, in pump operating time, the amount of leakage can be suppressed to substantially zero and the load required for the pressing member 46 to compress the discharging tube 27 can be configured to be the same as a load required to compress one discharging tube 27. As a result, it is possible to reduce the pump torque and variation in the pump load.

MODIFIED EXAMPLE

The above-described embodiment may be modified into the following forms.

The tube overlapping portion B may be created with reduced hardness so that the upstream portion 36 a and the downstream portion 36 b can be configured to have lower hardness than other portions of the discharging tube 27. Accordingly, the tube overlapping portion B may be more flexible other portions of the discharging tube 27. As a result, it is possible to reduce the load required for the pressing member 46 to compress the tube overlapping portion B.

The frictional resistance of the cam surfaces C of the pump foil 32 may vary depending on specifications of the tube pump 29. Accordingly, the pressing member 46 can appropriately press the discharging tube 27 or switch between the pump operating position and the pump non-operating position. As a result, it is possible to reduce the pump torque and improve a pump efficiency. Moreover, when the print head 19 is cleaned, the tube pump 29 can remove the liquid of the cam 24 effectively and stably.

The shaft sliding contact portions N of the cam surfaces C of the pump foil 32 may be formed of a low friction member made of polytetrafluoroethylene, polyacetal or polystyrene of sliding grade, or the like. In this case, the low friction member is mounted in the pump foil 32, as a molded product. Accordingly, it is possible to reliably reduce a sliding contact resistance between the shaft 45 and the shaft sliding contact portions N of the cam surfaces C when the pressing member 46 compresses the middle portion 36 of the discharging tube 27.

The shaft sliding contact portions N of the cam surfaces C of the pump foil 32 may be coated with a coating material with a low friction such as polytetrafluoroethylene or grease in order to reduce frictional resistance.

Accordingly, it is possible to reliably reduce the sliding contact resistance between the shaft 45 and the shaft sliding contact portions N of the cam surfaces C when the pressing member 46 compresses the discharging tube 27.

A surface processing such as grinding may be performed on the shaft sliding contact portions N of the cam surfaces C of the pump foil 32 in order to reduce the frictional resistance of the shaft sliding contact portions N.

An O-shaped ring or the like may be used instead of the separation preventing pin 47 as the separation prevention mechanism.

The roller 44 and the shaft 45 of the pressing member 46 may rotate together by inserting the shaft 45 into the shaft inserting hole 44 a of the roller 44.

The shaft 45 of the pressing member 46 may be formed of a low-friction material such as polytetrafluoroethylene.

The portions (both the ends) of the shaft 45 of the pressing member 46 that come in sliding contact with the cam surfaces C may be formed of a low-friction material such as polytetrafluoroethylene, metal, or polyacetal, polystyrene, or the like of sliding grade.

The roller 44 and the shaft 45 constituting the pressing member 46 may be integrated into a single unit.

The upstream concave portion 38 and the downstream concave portion 39 in the housing 31 may be arranged so as not to be adjacent in the inner circumferential surface 31 b of the housing 31.

The upstream concave portion 38 and the downstream concave portion 39 may be omitted from the housing 31.

As the pressing member 46, another sliding member that is slid to press the midway portion 36 of the discharging tube 27 may be used instead of the roller 44 and the shaft 45.

In the embodiment, an ink jet printer 11 is used as an example of a liquid ejecting apparatus. However, other liquid ejecting apparatus used to manufacture a color filter such as a liquid crystal display or to form pixels of an organic EL display or the like may be used. Alternatively, the tube pump 29 may be mounted in apparatuses other than liquid ejecting apparatuses. 

1. A tube pump comprising: a housing with cylindrical inner surface wherein a portion of a flexible tube is wound along the cylindrical inner surface of the housing with an upstream portion and a downstream portion of the tube partially overlapping at a tube overlapping portion, the housing including a pressing member for compressing the portion of the tube along the inner circumferential surface; wherein the housing includes a relief portion for releasing either the upstream portion or the downstream portion from the pressing of the pressing member located along the cylindrical inner surface at a position which corresponds to the location of the overlapping tube.
 2. The tube pump according to claim 1, wherein the relief portion includes an upstream relief portion and a downstream relief portion which correspond to the upstream portion and the downstream portion of the tube overlapping portion.
 3. The tube pump according to claim 2, wherein the upstream relief portion and the downstream relief portion are formed in the inner circumferential surface.
 4. The tube pump according to claim 1, wherein the overlapping portion of the flexible tube is more flexible than other portions of the flexible tube.
 5. The tube pump according to claim 1, further comprising a rotating body which is capable of rotating about an axis passing through the center of the housing, wherein the rotating body has a cam surface that comes into contact with the pressing member when the pressing member moves between a pump operating position wherein the tube is compressed in order to generate a negative pressure and a pump non-operating position wherein the tube is compressed so as not to generate the negative pressure, and wherein the frictional resistance of the cam surface varies.
 6. The tube pump according to claim 5, wherein the pressing member has a shaft that comes into contact with the cam surface of the rotating body, and wherein the frictional resistance of the portion of the shaft which comes in sliding contact with the cam surface is lower than other portions of the shaft.
 7. A liquid ejecting apparatus comprising: a liquid ejecting head which is capable of ejecting a liquid from a nozzle formed on a nozzle formation surface; a cap which is capable of sealing the nozzle formation surface; and a sucking mechanism which is capable of sucking the liquid from the cap, wherein the sucking mechanism includes the tube pump according to claim
 1. 8. A liquid ejecting apparatus comprising: a liquid ejecting head which is capable of ejecting a liquid from a nozzle formed on a nozzle formation surface; a cap which is capable of sealing the nozzle formation surface; and a sucking mechanism which is capable of sucking the liquid from the cap, a flexible tube and tube pump located near the middle of the flexible tube; wherein the tube pump comprises a housing with cylindrical inner surface wherein a portion of the flexible tube is wound along the cylindrical inner surface of the housing with an upstream portion and a downstream portion of the tube partially overlapping at a overlapping portion, and a pressing member for compressing the portion of the tube along the inner circumferential surface; wherein the housing includes a relief portion including a upstream relief portion and a downstream relief portion for releasing either the upstream portion or the downstream portion from the pressing of the pressing member, the relief portions being located along the cylindrical inner surface at positions which correspond to the location of the upstream portion and downstream portion of the overlapping portion.
 9. The apparatus according to claim 8, wherein the overlapping portion of the flexible tube is more flexible than other portions of the flexible tube.
 10. The apparatus according to claim 8, further comprising a rotating body which is capable of rotating about an axis passing through the center of the housing, wherein the rotating body has a cam surface that comes into contact with the pressing member when the pressing member moves between a pump operating position wherein the tube is compressed in order to generate a negative pressure and a pump non-operating position wherein the tube is compressed so as not to generate the negative pressure, and wherein the frictional resistance of the cam surface varies.
 11. The apparatus according to claim 10, wherein the pressing member has a shaft that comes into contact with the cam surface of the rotating body, and wherein the frictional resistance of the portion of the shaft which comes in sliding contact with the cam surface is lower than other portions of the shaft. 